1. Field of the Invention
This invention relates to a process for the preparation of polyamide compounds from polycarboxylic acids and diisocyanates.
2. Description of the Prior Art
It is already known that polyamides can be prepared from dicarboxylic acids and diisocyanates. It is also known that polyamide acids can likewise be prepared from tricarboxylic or tetracarboxylic acids and diisocyanates and such polyamide acids can be used as precursors to form imide linkages by ring closure through intramolecular dehydration and thereby product polyamide imides or polyimides. In the preparation of polyamides, polyamide acids, or polyamide acids containing imide linkages in a part of the molecular chain by utilizing the aforesaid reactions, it is a common procedure that a polycarboxylic acid react with a polyisocyanate in an organic polar solvent at a temperature ranging from room temperature to 250.degree. C. for a period of 1 to 20 hours. However, the reaction products thus obtained are usually lower in molecular weight than the polyamides prepared from a diamine and a dibasic acid dihalide or the polyamide acids prepared from a diamine and a tricarboxylic acid anhydride monohalide or a tetracarboxylic acid dianhydride. Moreover, a branched or cross-linked polymer tends to be formed, therefor such problems like an increase in the melt viscosity of the polymer, a decrease in the solubility of the polymer, and the like are often encountered. This makes it difficult to produce a linear, high-molecular-weight polymer suitable for use in melt processing or solution processing. Consequently, the uses of the polymers prepared by the aforesaid process are confined chiefly to varnishes, paints, adhesives and the like, and they can hardly be made into various forms requiring excellent physical properties, such as fibers, molded articles and films.
A number of processes for the preparation of a solution of a scarcely branched or cross-linked polymer by reacting a polycarboxylic acid with a polyisocyanate are also known. One exemplary process, which is disclosed in Japanese Patent Publication Nos. 676/'67 and 677/'67, comprises reacting an organic tetracarboxylic acid dialkyl ester with an organic diisocyanate. However, this process is not satisfactorily effective in producing a high-molecular-weight polymer. Moreover, a process for the preparation of a polyamide imide from an imide ring-containing dicarboxylic acid and a diisocyanate is known. This process has the disadvantage that a high-molecular-weight polymer is difficult to be produced or the resulting polymer is liable to gelation. Thus, the polymers prepared by this process are not suitable for the manufacture of satisfactory molded articles.
The catalysts commonly used for reaction with isocyanates are described in a textbook by J. H. Saunders et al., "Polyurethanes Chemistry and Technology", Vol. 1, pp. 228-232, and an article by J. W. Britain et al., J. Applied Polymer Science, Vol. 4, pp. 207-211 (1960). They include, for example, tertiary amines such as triethylamine, triethylenediamine, etc., alkali metal salts such as lithium acetate, sodium oleate, etc., metal alkoxides such as sodium methylate, etc., heavy metal salts such as cobalt naphthenate, cobalt benzoate, etc., and the like. Accordng to the present inventors' investigation, these catalysts were found to have only a minor effect on the reaction of a dicarboxylic acid with a diisocyanate. Moreover, when they were used for the reaction of a tricarboxylic acid and/or a tetracarboxylic acid with a diisocyanate, it proved difficult not only to produce a high-molecular-weight polyamide acid but also to prevent the resulting polymer from gelling in the course of the reaction. Further, the catalysts available for the preparation of polyamides, polyamide imides, polyimides and the like by the reaction of a carboxylic acid or an acid anhydride with an isocyanate include, for example, metal alkoxides as disclosed in U.S. Pat. Nos. 4,001,186, 4,061,622 and 4,061,623; lactamates as disclosed in U.S. Pat. Nos. 4,021,412, 4,094,864 and 4,094,866; cyclic phosphorus oxides as disclosed in U.S. Pat. No. 4,156,065; and the like. However, these compounds failed to manifest a sufficient degree of catalytic effect to produce a high-molecular-weight polyamide compound without causing the gelation of the polymer.